This invention relates to melting of glassmaking batch materials on a molten metal support, in particular, on molten copper alloys.
In a conventional glass melting process, glass batch materials are deposited onto a pool of molten glass where they float on the surface. Heat for melting is usually provided by overhead flames and, therefore, the great majority of heat for melting is imparted to the batch materials from the upper side only, which is relatively inefficient. Limits on the thermal durability of the roof of such a melting furnace restricts the amount of thermal input to the furnace and, thus, can limit the throughput of the furnace.
For these reasons, it has been proposed that glass be melted on a molten metal support wherein the high thermal conductivity of the molten metal would provide greater amounts of thermal energy to the underside of the batch layer and thereby provide a more efficient process. A molten metal support for melting is also advantageous for the sake of reducing the area of contact between the molten glass and ceramic containment elements which can contaminate the glass. Another advantage of melting on a molten metal support is that the glass melt may be maintained as a relatively thin layer rather than the deep pool maintained in a conventional tank type glass furnace. As a result, the furance may be reduced in size, less energy is consumed in maintaining the smaller volume of glass in a molten state, and color changes are made easier. Examples of molten metal supports for melting glass may be found in U.S. Pat. Nos. 3,450,516 (Emhiser et al.) and 3,764,287 (Brocious). The former discloses tin, gold or silver as the molten support, and the latter discloses the use of tin.
Gold and silver are obviously economically impractical to employ as molten metal supports in large scale commercial glass melting operations, but an element that is less expensive than tin and which possesses advantages over tin is copper. Potentially, copper could be less reactive to the glass compared to tin because of the lower reducing potential of copper. Also, the higher boiling point of copper entails a lower vapor pressure at a given temperature compared to tin at the same temperature. However, a major drawback of copper is its strong tendency to color glass. Cupric ions yield blue colored glass, and cuprous ions result in the development or ruby red coloration upon subsequent heat treatment. It would be desirable to control the coloration effects of copper so that molten copper could be used as a support for melting glass.
The use of copper alloyed with tin or other metals has been suggested for use as the molten metal support in the float process wherein molten glass is delivered onto a pool of molten metal and spreads to form a flat sheet, e.g., U.S. Pat. No. 710,357 (Heal). The temperatures involved in the float process, however, are considerably lower than those required for melting glass, and thus, infusion of unwanted coloring elements into the glass is less of a concern in the float process than in a melting process. In addition to higher temperatures a melting operation involves localized concentrations of alkalinity that promote chemical activity of the support metal. For these reasons, molten copper is considerably more difficult to employ as a support in a melting environment than in a float environment. Furthermore, for float forming, major proportions of copper are not employed because of its high melting temperature.
U.S. Pat. No. 3,127,261 (Long) discloses the use of a molten copper-containing alloy as a heat transfer medium for cooling molten glass in a conditioning section of a glass melter downstream from the melting zone.
In U.S. Pat. No. 3,670,179 (Loukes et al.), glass forming elements are reacted within a molten metal pool that may include copper alloys so as to synthesize a glass. The glass synthesis method is carried out at temperatures below those required for fusion melting of glass.
Preventing migration of tin into a glass ribbon being formed in a float process by incorporating trace amounts of various elements into the molten is disclosed in U.S. Pat. Nos. 3,305,337 (Loukes et al.); 3,337,323 (Loukes et al.); and 3,954,432 (Hummel et al.).